Anatomy of a Mill (Stamps)


The processes of mill work can be separated into four distinct stages. The first stage involves breaking down the copper bearing rock from the mill into small pieces – a process known as stamping. These small particles of rock are then passed onto a series of roughing jigs followed by a series of refining jigs. In the last stage the remaining rock is sent to the wash floor, where a series of slime tables are used to remove the last traces of copper from the rocks before being discarded. We start today with the first and most important step of the entire process: the stamps.

A stamp is essentially an industrial sized mortar and pestle, used to perform two essential functions. One of these is to simply pulverize the copper-bearing rock into smaller and more manageable pieces for use by the mill’s remaining machinery. A stamp’s primary job, however, is to break off any useless rock attached to the copper itself. The stamping process accomplished this thanks to copper’s malleableness in contrast to the surrounding rocks extreme brittleness. With each blow of a stamp head, copper will merely deform while the rock surrounding it will break apart.

For many decades these blows were accelerated by gravity alone, in machines known commonly as Gravity Stamps. By the middle of the 19th century a new type of stamp was slowly making its way to the Copper Country: the steam stamp. These stamps used steam to drive the stamp down onto the rock – creating a much more forceful blow and increased efficiency. A battery of just 3-4 steam stamps could do the work of almost a dozen gravity stamps. A revolution in milling technology had begun.

Several types of stamps were built, most of which differed only slightly in design and implementation. The most popular Copper Country stamps were the Ball, Leavitt and Allis types, with the Ball quickly becoming phased out by the turn of the century. For the sake of this overview we’ll be concentrating on the Allis Stamp, an illustration of which can be seen above courtesy of the “Ore Dressing Textbook” by Robert Richards. These stamps – like all stamps – consisted of five major parts: the Mortar Blocks, the Mortar, the Stamp, the Cylinder, and the frame.

Mortar Blocks

Early stamps – especially the Ball type – required a very extensive foundation to work properly. Stamps could not simply be bolted down to a mill’s foundation, the repetitive force from the stamp head would shake a mill apart. In order to cushion a mill’s foundation from these blows, a series of pine timbers laid at right angles to each other would be placed under the stamp itself. These were known as the Mortar Blocks. Unfortunately these large timbers – usually white pine – were expensive and required replacement every few months. Modern stamps such as the Allis replaced the blocks with a large cast-iron base which adequately softened the blow to the concrete foundation. These “blocks” are visible in the photo above from the Quincy Mill.

The Mortar

The Mortar is essentially the surface on which the rock is smashed. In the center of the mortar, directly under the stamp head, is set an extremely strong piece of forged iron known as a die. The die was usually a flat round piece, held in place by a series of wedges set around its perimeter. Since it received the majority of the force from the stamp, the die would wear down over time and needed to be replaced about every 16-24 months.

Sitting atop of the Mortar is an enclosed steel box known at the Mortar Box. This box serves as the container for the rock being stamped as well as a conduit through which the water used in the process is pumped. One end of the Mortar box features an opening in which rock is loaded, while the other end is covered by a steel screen perforated with a series of small holes. As the stamp works against the rocks, water is pumped into the box and out through the screen. Once the rock has been pulverized down into a small enough size, it too passes through the screen and on to the next stage.

In an Allis stamp the screen is in fact several small screens which are set into a series of frames at the end of the Mortar Box. As these screens wear down they are replaced, after being flipped over to wear the opposite end for a while. The screen frames can be flipped up to allow strangling pieces of rock to be swept out of the box every few hours, as well as providing access to the stamp head and die for replacement purposes.

The Stamp

Serving as the pestle in our analogy, the stamp consists of a long shaft tipped by a heavy iron head known as a shoe. A typical shoe weighs around 800 lbs and is driven downward at over 25 feet per second to create nearly 4 tons of downward force. This force wears down the stamp shoes considerably, requiring them to be replaced every few weeks.

The shoe itself is enclosed within the Mortar Box, with the stamp shaft protruding through a hole in the box’s top. After each downward stroke (nearly 100 every minute) the shoe is then raised up about two feet until it just emerges from the water within the Mortar Box before being dropped again. The resulting splash as the stamp enters the water helps to stir up the Mortar Box’s contents and push rock through the screens.

In addition to being raised and dropped, the stamp is also turned once every four blows, to insure even wear on the shoe. Groove run along opposite sides of the stamp shaft, which are used to attach a ring halfway up the shaft. The ring is turned by means of an attached belt and pulley, which derives power from an external source. (you can see the pulley and belt in the photo above, on the left. Its also marked with an arrow in the last photo of this post)


The cylinder is the driving force behind the stamp, and is what differentiates it from an old-style gravity stamp. A typical Allis stamp cylinder features a 20 inch bore and a maximum stroke of 24 inches. Inside the cylinder is a piston, which is connected to the stamp shaft. The piston is acted upon from both ends in succession, first from the top to drive it downward and then from the bottom to raise it back up. Two separate valve assemblies (seen in the photo above) deliver high pressure steam to both the upper and lower chambers of the cylinder. These valves are controlled and timed by a series of cams and rods powered by an external belt drive.

With the stamp in its highest position steam is allowed to enter the upper chamber of the cylinder, forcing the piston and connected stamp downward. Steam is then allowed to enter the lower chamber of the cylinder, which forces the piston and stamp upward. During this time steam has been allowed to exit the upper chamber. Just before the piston is to reach its upper limit steam is allowed to enter the upper chamber yet again – coushing the piston and stopping it before it hits the top of the cylinder. More steam is then allowed in as the process repeats.

The Frame

The frame of a stamp consists of three sections which each support different components of the machine. At the bottom the frame supports the Mortar Box, as well as the water feed pipes and rock chutes. In the middle the frame supports the stamp shaft through a set of guides. At the top the frame supports the cylinder and valve assemblies including the drive mechanism for the valves. A series of angles braces keeps the whole thing upright.

Describing a stamp and showing archive photos of one is one thing, but seeing it in person is something completely different. Next up we’ll take a closer look at a real life example – the only steam stamp still in existence in the Copper Country…

Some photos used in the series are courtesy the Library of Congress Prints and Photographs Division, HAER. Information and illustrations obtained from “Ore Dressing” by Robert Richards. Other information obtained from “The Copper Mines of Lake Superior” by T.A. Rickard.

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  1. There are two things I found very interesting about Stamps when doing the research for this post. First of which was the need for an independent power source to power the valves for the cylinder. For some reason (ignorance mostly) I had assumed that the stamps required no power to operate – only the steam itself. Not true. This means that a mill required a steam engine of its own somewhere on site to run the belt drive for these valves.

    That power source also had to twist the stamps, which is the second thing that I found interesting. In all the Quincy pics I noticed that pulley attached to the shaft and wondered what it was for – crazy. And to think that all this technology was designed and built over a century and a half ago.

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